Solar cell and an arrangement and a method for producing a solar cell

ABSTRACT

The present invention relates generally to solar cells, material layers within solar cells, a production method of solar cells, and a manufacturing arrangement for producing solar cells. A solar cell according to the invention includes at least one layer with a surface, produced by laser ablation, wherein the uniform surface area to be produced includes at least an area 0.2 dm 2  and the layer has been produced by employing ultra short pulsed laser deposition wherein pulsed laser beam is scanned with a rotating optical scanner including at least one mirror for reflecting the laser beam.

FIELD OF INVENTION

The present invention relates generally to solar cells, material layerswithin solar cells, a production method of solar cells, and amanufacturing arrangement for producing solar cells. More specifically,the present invention relates to what is disclosed in the preamble ofthe independent claim.

BACKGROUND

Solar cells provide an ecological way of producing energy, and they havetherefore been under intensive development. Solar cells are generallymade of photovoltaic cells. A photovoltaic cell has a at least onesemiconducting layer, wherein photons of light are absorbed. Theabsorption of light causes electrons or holes to transfer to a higher,conducting energy level, and this energy can be used as electricity.

In order to conduct the formed electricity out of the solar cells theremust be a conducting layer at the both sides of the semiconductinglayer(s). The conducting layer at the radiated surface of the solar cellmust allow light to enter the semiconductor layer. A solar cell isusually partitioned into small cells which are connected in series orparallel. In this case the conducting layers and possibly thesemiconductor layer(s) are patterned to provide the required circuitry.

The radiated surface of the solar cell is further coated with one orseveral layers for providing antireflection surface for the solar cell,and for protecting the solar cell from mechanical, chemical and physicalstresses of the environment. Such surfaces may be radiation stabilizedglass or plastic products. For example, a glass layer may includeself-cleaning TiO₂ coating made by sputtering,Hot-Aerosol-Layering-Operation (nHALO) and by ALD-techniques. Theoutermost protecting layers may be integrated in the solar cell or theycan be separated from the electrical layers.

At the time of filing this patent application there are two maintechnologies used for producing solar cells. In the first technologysilicon or some other semiconducting material is used as a substrate,and further layers are provided on the substrate. So far, thistechnology is most generally used. However, the production of thesilicon substrates as well as forming the further layers with thepresent manufacturing technology is costly. Also, the weight of largesolar cells becomes high, and the large solar cells are relativelysensitive to mechanical strains. These disadvantages prevent anincreased use of solar cells.

Another technology for producing solar cells is based on using someother substrate and producing the semiconducting layers as well as otherlayers as films on the substrate. The substrate may be e.g. glass orplastic. The substrate can be used as the radiated surface of the solarcell, in which case the substrate is made transparent. The solar cellsmade with this technology have less weight and are not as sensitive tomechanical stresses. However, it has been a problem to achievesufficient efficiency; usually less than 10% of the energy of light canbe converted into electrical energy. One reason for this is thenon-homogeneity of the produced layers. Therefore the transparency ofthe radiated layers may be insufficient. Also, the non-homogeneity ofsemiconducting layers causes loss of energy.

One problem which causes lack of efficiency is the fact that a junctionof semiconducting layer has a specified electric potential threshold,and the energy of photons can only be converted into an amount of energywhich corresponds to the potential threshold. The solar light has a widespectrum of wavelengths and thus the photons have a wide range ofenergies. If the energy of a photon is lower than the thresholdpotential of the semiconducting junction, the photon is not convertedinto electrical energy. On the other hand, if the energy of a photon ishigher than the threshold potential of the semiconducting junction, thephoton is converted into energy according to the potential threshold,but the energy of the photon above the threshold is converted into heat.

The problem of converting wide spectrum radiation into electrical energycould be solved by providing several successive, transparentsemiconductor layers wherein each pair of semiconducting layers providesa junction for converting light to electricity. The semiconductorjunctions nearest to the radiated surface have the highest thresholdpotential and the threshold potential decreases when the radiationenters the following junctions. This way a photon is converted intoelectricity at a junction which has a threshold potential close to thephoton energy, and high efficiency could be achieved. However, it isdifficult manufacture several successive, transparent semiconductinglayers. If the surfaces of the layers are not sufficiently smooth, lightis reflected at each junction layers thus decreasing the efficiency.Also, the non-homogeneity of several semiconductor layers causes loss ofelectrical energy due to e.g. short circuiting spots in the layer andother uneven distribution of electrical fields.

Further, the production of protecting layers at the radiated surface ofa solar cell is also difficult and costly. The processes are slow andneed to be made separately from producing the electrical part of thesolar cell. Handling the parts in different processes and/or assemblyphases may involve a risk of contamination and thus a further loss ofefficiency of the final product.

The above mentioned problems become all much more difficult whenproducing large solar cells as it is necessary to produce layers oflarge surfaces. The known technologies are suitable for producing cellsof small dimensions, e.g. areas of a few cm² at the most, but thequality of surfaces and homogeneity of materials become worse if knowntechnologies would be applied in producing solar cells with layers oflarge surfaces.

The applicant has investigated possibilities for using laser coldablation in production of solar cells. In the recent years, considerabledevelopment of the laser technology has provided means to produce veryhigh-efficiency laser systems that are based on semi-conductor fibres,thus supporting advance in so called cold ablation methods. Coldablation is based on forming high energy laser pulses of short duration,such as within picosecond range, and directing the pulses into thesurface of a target material. A plume of plasma is thus ablated from thearea where the laser beam hits the target. The applications of coldablation include e.g. coating and machining.

When employing novel cold-ablation, both qualitative and production raterelated problems associated with coating, thin film production as wellas cutting/grooving/carving etc. has been approached by focusing onincreasing laser power and reducing the spot size of the laser beam onthe target. However, most of the power increase was consumed to noise.The qualitative and production rate related problems were stillremaining although some laser manufacturers resolved the laser powerrelated problem. Representative samples for both coating/thin film aswell as cutting/grooving/carving etc. could be produced only with lowwith repetition rates, narrow scanning widths and with long working timebeyond industrial feasibility as such, highlighted especially for largebodies.

Because the energy content of a pulse, the power of the pulse increasesin the decrease of the pulse duration, the problem significanceincreases with the decreasing laser-pulse duration. The problems occursignificant even with the nano-second-pulse lasers, although they arenot applied as such in cold ablation methods.

The pulse duration decrease further to femto or even to atto-secondscale makes the problem almost irresolvable. For example, in apico-second laser system with a pulse duration of 10-15 ps the pulseenergy should be 5 μJ for a 10-30 μm spot, when the total power of thelaser is 100 W and the repetition rate 20 MHz. Such a fibre to toleratesuch a pulse is not available at the priority date of the currentapplication according to the knowledge of the writer at the very date.

The prior art laser treatment systems most often include opticalscanners which are based on vibrating mirrors. Such an optical scanneris disclosed in e.g. document DE10343080. A vibrating mirror oscillatesbetween two determined angles relative to an axis which is parallel tothe mirror. When a laser beam is directed to the mirror, it is reflectedwith an angle which depends on the position of the mirror at thatmoment. The vibrating mirror thus reflects or “scans” the laser beaminto points of a line at the surface of a target material.

An example of a vibrating scanner or “galvano-scanner” is illustrated inFIG. 1 a. It has two vibrating mirrors, one of which scans the beamrelative to X-axis and another scans the beam relative to orthogonaly-axis.

The production rate is directly proportional to the repetition rate orrepetition frequency. On one hand the known mirror-film scanners(galvano-scanners or back and worth wobbling type of scanners), which dotheir duty cycle in a way characterized by their back and forthmovement, the stopping of the mirror at the both ends of the duty cycleis somewhat problematic as well as the accelerating and deceleratingrelated to the turning point and the related momentary stop, which alllimit the usability of the mirror as scanner, but especially also to thescanning width. The present coating methods employing galvano-scannerscan produce scanning widths at most 10 cm, preferably less. If theproduction rate were tried to be scaled up, by increasing the repetitionrate, the acceleration and deceleration cause either a narrow scanningrange, or uneven distribution of the radiation and thus the plasma atthe target when radiation hit the target via accelerating and/ordecelerating mirror.

Conventionally galvanometric scanners are used to scan a laser beam witha typical maximum speed of about 2-3 m/s, in practice about 1 m/s. Iftrying to increase the coating/thin film production rate by simplyincreasing the pulse repetition rate, the present above mentioned knownscanners direct the pulses to overlapping spot of the target areaalready at the low pulse repetition rates in kHz-range, in anuncontrolled way. With repetition rate of 2 MHz even 40-60 successivepulses are overlapping. The overlapping of spots 111 in such a situationare illustrated in FIG. 1 b.

At worst, such an approach results in release of particles from thetarget material, instead of plasma but at least in particle formationinto plasma. Once several successive laser pulses are directed into thesame location of target surface, the cumulative effect seems to erodethe target material unevenly and can lead to heating of the targetmaterial, the advantages of cold ablation being thus lost.

The same problems apply to nanosecond range lasers, the problem beingnaturally even more severe because of the long lasting pulse with highenergy. Here, the target material heating occurs always, the targetmaterial temperature being elevated to approximately 5000 K. Thus, evenone single nanosecond range pulse erodes the target materialdrastically, with aforesaid problems.

In the known techniques, the target may not only ware out unevenly butmay also fragment easily and degrade the plasma quality. Thus, thesurface to be coated with such plasma also suffers the detrimentaleffects of the plasma. The surface may comprise fragments, plasma may benot evenly distributed to form such a coating etc. which are problematicin accuracy demanding application, but may be not problematic, withpaint or pigment for instance, provided that the defects keep below thedetection limit of the very application.

The present methods ware out the target in a single use so that sametarget is not available for a further use from the same surface again.The problem has been tackled by utilising only a virgin surface of thetarget, by moving target material and/or the beam spot accordingly.

In machining or work-related applications the left-overs or the debriscomprising some fragments also can make the cut-line uneven and thusinappropriate, as the case could for instance in flow-control drillings.Also the surface could be formed to have a random bumpy appearancecaused by the released fragments, which may be not appropriate inmanufacturing of solar cells.

In addition, the mirror-film scanners moving back and forth generateinertial forces that load the structure it self, but also to thebearings to which the mirror is attached and/or which cause the mirrormovement. Such inertia little by little may loosen the attachment of themirror, especially if such mirror were working nearly at the extremerange of the possible operational settings, and may lead to roaming ofthe settings in long time scale, which may be seen from unevenrepeatability of the product quality. Because of the stoppings, as wellas the direction and the related velocity changes of the movement, sucha mirror-film scanner has a very limited scanning width so to be usedfor ablation and plasma production. The effective duty cycle isrelatively short to the whole cycle, although the operation is anywayquite slow. In the point of view of increasing the productivity of asystem utilising mirror-film scanners, the plasma making rate is inprerequisite slow, scanning width narrow, operation unstable for longtime period scales, but yield also a very high probability to getinvolved with unwanted particle emission in to the plasma, andconsequently to the products that are involved with the plasma via themachinery and/or coating.

The product lifetime of solar cells should also be increased and themaintenance costs should be lowered, sustainable development being aprerequisite. The production of layers especially uniform layer surfacesof large solar cell with one or several of the following properties:excellent optical transparency, chemical and/or wear resistance,scratch-free surface, thermal resistance, coating adhesion,self-cleaning properties and properties derived from resistivity haveremained an unsolved problem.

Neither recent high-technological coating methods, nor present coatingtechniques related to laser ablation either in nanosecond or coldablation range (pico-, femto-second lasers) can provide any feasiblemethod for industrial scale coating of glass products comprising largersurfaces. The present CVD- and PVD-coating technologies requirehigh-vacuum conditions making the coating process batch wise, thusnon-feasible for industrial scale production of solar cells. Moreover,the distance between the material to be coated and the coating materialto be ablated is long, typically over 50 cm, making the coating chamberslarge and vacuum pumping periods time- and energy-consuming. Suchhigh-volume vacuumed chambers are also easily contaminated with coatingmaterials in the coating process itself, requiring continuous andtime-consuming cleaning processes.

While trying to increase the production rate in present laser-assistedcoating methods, various defects such as short circuiting defectfactors, pinholes, increased surface roughness, decreased ordisappearing optical transparency, particulates on layer surface,particulates in surface structure affecting corrosion pathways,decreased surface uniformity, decreased adhesion, etc. take place.

Plasma related quality problems are demonstrated in FIGS. 2 a and 2 b,which indicate plasma generation according to known techniques. A laserpulse 214 hits a target surface 211. As the pulse is a long pulse, thedepth h and the beam diameter d are of the same magnitude, as the heatof the pulse 214 also heat the surface at the hit spot area, but alsobeneath the surface 211 in deeper than the depth h. The structureexperiences thermal shock and tensions are building, which whilebreaking, produce fragments illustrated F. As the plasma may be in theexample quite poor in quality, there appears to be also molecules andclusters of them indicate by the small dots 215, as in the relation tothe reference by the numeral 215 for the nuclei or clusters of similarstructures, as formed from the gases 216 demonstrated in the FIG. 2 b.The letter “o”s demonstrate particles that can form and grow from thegases and/or via agglomeration. The released fragments may also grow bycondensation and/or agglomeration, which is indicated by the curvedarrows from the dots to Fs and from the os to the Fs. Curved arrowsindicate also phase transitions from plasma 213 to gas 216 and furtherto particles 215 and increased particles 217 in size. As the ablationplume in FIG. 2 b can comprise fragments F as well as particles built ofthe vapours and gases, because of the bad plasma production, the plasmais not continuous as plasma region, and thus variation of the qualitymay be met within a single pulse plume. Because of defects incomposition and/or structure beneath the deepness h as well as theresulting variations of the deepness (FIG. 2 a), the target surface 211in FIG. 2 b is not any more available for a further ablations, and thetarget is wasted, although there were some material available.

SUMMARY OF THE INVENTION

An object of the present invention is to provide solar cells, as well asan arrangement and method of their production, wherein the describeddisadvantages of the prior art are avoided or reduced.

The object of the invention is therefore to provide a technology forproducing layers with certain surface by pulsed laser deposition that sothat the uniform surface area to be coated comprises at least 0.2 dm².

A second object of this invention is to provide new solar cell productswherein layers are produced by pulsed laser deposition so that theuniform surface area of the layer comprises an area of at least at least0.2 dm².

A third object of this invention is to solve a problem how to provideavailable such fine plasma practically from a target to be used in solarcell products, so that the target material do not form into the plasmaany particulate fragments either at all, i.e. the plasma is pure plasma,or the fragments, if exist, are rare and at least smaller in size thanthe ablation depth to which the plasma is generated by ablation fromsaid target.

A fourth object of the invention is to provide at least a new methodand/or related means to solve how to provide the uniform surface area ofa layer in a solar cell product with the high quality plasma withoutparticulate fragments larger in size than the ablation depth to whichthe plasma is generated by ablation from said target, i.e. to coatsubstrates with pure plasma.

A fifth object of this invention is to is to provide a good adhesion ofthe coating to the uniform surface area of a glass product by said pureplasma, so that wasting the kinetic energy to particulate fragments issuppressed by limiting the existence of the particulate fragments ortheir size smaller than said ablation depth. Simultaneously, theparticulate fragments because of their lacking existence in significantmanner, they do not form cool surfaces that could influence on thehomogeneity of the plasma plume via nucleation and condensation relatedphenomena.

A sixth object of the invention is to provide at least a new methodand/or related means to solve a problem how to provide a broad scanningwidth simultaneously with fine plasma quality and broad coating widtheven for large solar cell bodies in industrial manner.

A seventh object of the invention is to provide at least a new methodand/or related means to solve a problem how to provide a high repetitionrate to be used to provide industrial scale applications in accordancewith the objects of the invention mentioned above.

An eighth object of the invention is to provide at least a new methodand/or related means to solve a problem how to provide good qualityplasma for coating of uniform glass surfaces to manufacture solar cellproducts according to the first to seven objects, but still save targetmaterial to be used in the coating phases producing same qualitycoatings/thin films where needed.

A further object of the invention is to use such method and meansaccording previous objects to solve a problem how to cold-work and/orproduce layers of a solar cell.

The present invention is based on the surprising discovery that layersfor solar cell products comprising large surfaces can be produced withindustrial production rates and excellent qualities regarding one ormore of technical features such as optical transparency, chemical and/orwear resistance, scratch-free-properties, thermal resistance and/orconductivity, resistivity, coating adhesion, self-cleaning properties,particulate-free coatings, pinhole-free coatings and electronicconductivity by employing ultra short pulsed laser deposition in amanner wherein pulsed laser beam is scanned with a rotating opticalscanner comprising at least one mirror for reflecting said laser beam.Moreover, the present method accomplishes the economical use of targetmaterials, because they are ablated in a manner accomplishing the reuseof already subjected material with retained high coating results. Thepresent invention further accomplishes the producing of product layersin low vacuum conditions with simultaneously high coating properties.Moreover, the required coating chamber volumes are dramatically smallerthan in competing methods. Such features decrease dramatically theoverall equipment cost and increase the coating production rate. In manypreferable cases, the coating equipment can be fitted intoproduction-line in online manner.

More specifically, the object of the invention is achieved by providinga method for producing by laser ablation at least one layer having asurface and to be used as part of a solar cell, which is characterizedin that the surface area to be produced comprises at least an area 0.2dm² and the coating is performed by employing ultra short pulsed laserdeposition wherein pulsed laser beam is scanned with a rotating opticalscanner comprising at least one mirror for reflecting said laser beam.

The invention also relates to a solar cell comprising at least one layerwith a surface, produced by laser ablation, which is characterized inthat the uniform surface area to be produced comprises at least an area0.2 dm² and the layer has been produced by employing ultra short pulsedlaser deposition wherein pulsed laser beam is scanned with a rotatingoptical scanner comprising at least one mirror for reflecting said laserbeam.

Some embodiments of the invention are described in dependent claims.

In this patent application term “light” means any electromagneticradiation which can be used for cold ablation and “laser” means lightwhich is coherent or a light source producing such light. “Light or“laser” is thus not restricted in any way to the visible part of thelight spectrum.

In this patent application term “ultra short pulse laser deposition”means that a certain point at the target surface is radiated with alaser beam for a time period of less than 1 ns, preferably less than 100ps at a time. Such an exposure may be repeated at the same location ofthe target.

In this patent application term “coating” means forming material layerof any thickness on a substrate. Coating thus may also mean producingthin films with a thickness of e.g. <1 μm.

In this patent application term “surface” may mean surface of a layer,coating and/or, wherein the surface may be an outer surface or it mayform an interface with other layer/coating/substrate. The surface mayalso be a surface of a half-finished product, which may be furtherprocessed to achieve a final product.

BRIEF DESCRIPTION OF THE DRAWINGS

The described and other advantages of the invention will become apparentfrom the following detailed description and by referring to the drawingswhere:

FIG. 1 illustrates an exemplary galvano-scanner set-up employed in stateof the art cold ablation coating/thin-film production and in machiningand other work-related applications.

FIG. 1 b illustrates the situation wherein prior art galvanometricscanner is employed in scanning laser beam resulting in heavyoverlapping of pulses with repetition rate of 2 Mhz.

FIG. 2 a illustrates plasma-related problems of known techniques.

FIG. 2 b illustrates further plasma-related problems of knowntechniques.

FIG. 3 illustrates exemplary layers produced for a solar cell.

FIG. 4 illustrates an exemplary arrangement according to the inventionfor producing a layer for a solar cell using pulsed laser technology.

FIG. 5 illustrates an exemplary arrangement according to the inventionfor producing several layers of a solar cell using pulsed lasertechnology.

FIG. 6 a illustrates one possible turbine scanner mirror employed in amethod according to the invention,

FIG. 6 b illustrates the movement of the ablating beam achieved by eachmirror in the example of FIG. 6 a.

FIG. 7 illustrates beam guidance through one possible rotating scannerto be employed according to the invention.

FIG. 8 a illustrates beam guidance through another possible rotatingscanner to be employed according to the invention.

FIG. 8 b illustrates beam guidance through a further possible rotatingscanner to be employed according to the invention.

FIG. 10 a illustrates an embodiment according to the invention, whereintarget material ablated by scanning the laser beam with rotating scanner(turbine scanner).

FIG. 10 b illustrates an exemplary part of target material of FIG. 10 a.

FIG. 10 c illustrates an exemplary ablated spot of target material ofFIG. 110 b.

FIG. 11 illustrates an exemplary way according to the invention to scanand ablate target material with a rotating scanner.

DETAILED DESCRIPTION

FIGS. 1 a, 1 b, 2 a and 2 b where already described above in the priorart description.

FIG. 3 illustrates exemplary layers of a solar cell, which is based onfilm layer technology. The substrate 360 may be e.g. glass or plasticmaterial. At the radiated surface of the solar cell there is aantireflection layer 362. There may also be other or additional layersfor keeping the outer surface clean and protected from environmentalstresses. At the inner surface of the substrate 360 there is anelectrically conducting layer 364 which may be patterned according tothe circuit layout and partitioning of the solar cell. The conductinglayer is preferably transparent and/or the conductive wiring is made ofnarrow leads which cover only a small part of the surface area. Next,above the conductive layer, there is one or several semiconductinglayers 366. Finally, there is another conductive layer 368 for providingwiring of a second electrical potential of the solar sell. The secondconductive layer need not be transparent if there are no furthersemiconducting layers behind the conducting layer. There may also be anadditional, protecting layer at the surface of the second conductivelayer.

If a solar cell is produced with a semiconducting substrate, thereexists similar layers in similar order, but the production isimplemented by starting with a semiconducting substrate and producingother layers on that substrate.

FIG. 4 illustrates an exemplary system for treating material with laserablation. A laser beam formed by a laser source 44 and scanned with arotating optical scanner 10 towards the target. The target 47 has a formof a band which is spooled from a feed roll 48 into a discharge roll 46.The target is supported with a support plate 51 which has an opening 52at the location of ablation. However, the target may alternatively beother than band, such as a rotating cylinder of target material. Whenthe laser beam 49 received from the scanner hits the target, material isablated, and a plasma plume is provided. A substrate 50 is provided intothe plasma plume. The substrate will thus be coated with a layer oftarget material. If a layer is to be machined after the deposition, thiscan be made with a laser beam.

It is also to possible to provide the laser ablation with many otheralternative structures and arrangements. For example, it is naturallypossible to provide the deposition from under or above the substrate orboth. It is also possible to use target material which is provided on atransparent sheet. In such an arrangement it is possible to provide thetarget material very close to the substrate and to provide the laserbeam to the target material through the transparent part of the sheet.If the target material is a thin film at the sheet, it will ablatetowards the substrate. The target sheet can be first produced byablating the target material on a transparent sheet.

FIG. 5 illustrates an exemplary production line arrangement forproducing layers for a solar cell. The arrangement includes five laserprocessing units 571-575 within a same processing chamber 510. Above theprocessing unit there is a conveyor 591 for transferring substrates581-585 along the line. Each processing unit provides a certain processfor the substrate. The processing units may produce layers or they mayprovide laser machining of the substrate or produced layers. Naturally,there may also be other types of processing units within the productionline. It is an important advantage of the invention that layers ofdifferent materials can be deposited within a same chamber on the sameproduction line. It is even possible to provide possibly required laserpatterning. When all or most of the layers are produced in the samechamber, the risk of contamination or other defects due to handling ofsemi-finished products is minimal.

Next, the physical basis and structure of suitable rotating scanners isdescribed. According to the invention there is provided a method forproviding a layer for a solar cell with a certain surface by laserablation in which method the surface area to be coated comprises atleast 0.2 dm² and the depositing is carried by employing ultra shortpulsed laser deposition wherein pulsed laser beam is scanned with arotating optical scanner comprising at least one mirror for reflectingsaid laser beam.

Ultra Short Laser Pulsed Deposition is often shortened USPLD. Saiddeposition is also called cold ablation, in which one of thecharacteristic features is that opposite for example to competingnanosecond lasers, practically no heat transfer takes place from theexposed target area to the surroundings of this area, the laser pulseenergies being still enough to exceed ablation threshold of targetmaterial. The pulse lengths are typically under 50 ps, such as 5-30 ps.i.e. ultra short, the phenomena of cold ablation being reached withpico-second but also femto-second and atto-second pulsed lasers. Thematerial evaporated from the target by laser ablation is deposited ontoa substrate that can be held near room temperature. Still, the plasmatemperature reaches 1,000,000 K on exposed target area. The plasma speedis superior, gaining 100,000 m/s and thus, leading to better adhesion ofcoating/thin-film produced. In a more preferred embodiment of theinvention, said uniform surface area comprises at least 0.5 dm². In astill more preferred embodiment of the invention, said uniform surfacearea comprises at least 1.0 dm². The invention accomplishes easily alsothe coating of products comprising uniform coated surface areas largerthan 0.5 m², such as 1 m² and over. The process is especially beneficialfor coating large surfaces of layers for solar cells with high qualityplasma.

In industrial applications, it is important to achieve high efficiencyof laser treatment. In cold ablation, the intensity of laser pulses mustexceed a predetermined threshold value in order to facilitate the coldablation phenomenon. This threshold value depends on the targetmaterial. In order to achieve high treatment efficiency and thus,industrial productivity, the repetition rate of the pulses should behigh, such as 1 MHz, preferably over 2 MHz and more preferably over 5MHz. As mentioned earlier, it is advantageous not to direct severalpulses into same location of the target surface because this causes acumulating effect in the target material, with particle depositionleading to bad quality plasma and thus, bad quality coatings andthin-films, undesirable eroding of the target material, possible targetmaterial heating etc. Therefore, to achieve a high efficiency oftreatment, it is also necessary to have a high scanning speed of thelaser beam. According to the invention, the velocity of the beam at thesurface of the target should generally be more than 10 m/s to achieveefficient processing, and preferably more than 50 m/s and morepreferably more than 100 m/s, even such speeds as 2000 m/s.

FIG. 6 a illustrates an example of a rotating turbine scanner, which canbe used in implementing the invention. According to this embodiment,rotating optical scanner comprises at least three mirrors for reflectinglaser beam. In one embodiment of the invention, in the coating methodemploys a polygonal prism illustrated in FIG. 5. Here, a polygonal prismhas faces 21, 22, 23, 24, 25, 26, 27 and 28. Arrow 20 indicates that theprism can be rotated around its axis 19, which is the symmetry axis ofthe prism. When the faces of the prism of the FIG. 6 a are mirror faces,advantageously oblique in order to achieve scanning line, arranged suchthat each face in its turn will change, by means of reflection, thedirection of radiation incident on the mirror surface as the prism isrotated around its axis, the prism is applicable in the method accordingto an embodiment of the invention, in its radiation transmission line,as part of a rotating scanner, i.e. turbine scanner. FIG. 6 a shows 8faces, but there may be considerably more faces than that, even dozensor hundreds of them. FIG. 6 a also shows that the mirrors are at thesame oblique angle to the axis, but especially in an embodimentincluding several mirrors, the said angle may vary in steps so that, bymeans of stepping within a certain range, a certain stepped shift on thework spot is achieved on the target, illustrated in FIG. 6 b. Thedifferent embodiments of invention are not to be limited into variousturbine scanner mirror arrangements regarding for example the size,shape and number of laser beam reflecting mirrors.

The structure of the turbine scanner, FIG. 6 a, includes at least 2mirrors, preferably more than 6 mirrors, e.g. 8 mirrors (21 to 28)positioned symmetrically around the central axis 19. As the prism 21 inthe turbine scanner rotates 20 around the central axis 19, the mirrorsdirect the radiation, a laser beam, for instance, reflected from spot29, accurately onto the line-shaped area, always starting from one andthe same direction (FIG. 6 b). The mirror structure of the turbinescanner may be non-tilted (FIG. 7) or tilted at a desired angle, e.g.FIGS. 8 a and 8 b. The size and proportions of the turbine scanner canbe freely chosen. In one advantageous embodiment of the coating methodit has a perimeter of 30 cm, diameter of 12 cm, and a height of 5 cm.

In an embodiment of the invention it is advantageous that the mirrors 21to 28 of the turbine scanner are preferably positioned at oblique anglesto the central axis 19, because then the laser beam is easily conductedinto the scanner system.

In a turbine scanner according to be employed according to an embodimentof the invention (FIG. 6 a) the mirrors 21 to 28 can deviate from eachother in such a manner that during one round of rotational movementthere are scanned as many line-shaped areas (FIG. 6 b) 29 as there aremirrors 21 to 28.

According to one embodiment of the invention, rotating optical is heremeant scanners comprising at least one mirror for reflecting laser beam.Such a scanner and its applications are described in patent applicationF120065867. FIG. 9 illustrates a scanner 910 with one rotating mirror.The mirror 914 is arranged to rotate around the axis of rotation 916.FIG. 9 also shows the side view and the end view of the mirror. Themirror has a shape of a cylinder, which is slightly tilted in relationto the axis of rotation 916. The mirror is shown as a tilted cylinder inorder to better visualize the form of the mirror, and the ends of themirror are therefore oblique. However, it would also be possible to haveedges which are perpendicular to the axis of rotation. The opticalscanner has an axle at the axis of rotation, in which the mirror isconnected. The mirror may be connected to the rotating axle with e.g.end plates or spokes (not shown in the Figure).

FIG. 10 a demonstrates a target material ablated with pico-second-rangepulsed laser employing rotating scanner with speed accomplishing theablation of target material with slight overlapping of adjacent pulses,avoiding the problems associated with prior art galvano-scanners. FIG.10 b shows enlarged picture of one part of the ablated material, clearlydemonstrating the smooth and controlled ablation of material on both x-and y-axis and thus, generation of high quality, particle-free plasmaand further, high quality thin-films and coatings. FIG. 10 cdemonstrates one example of possible x- and y-dimensions of one singleablation spot achieved by one or few pulses. Here, it can be clearlyseen, that the invention accomplishes the ablation of material in amanner wherein the width of the ablated spot is always much bigger thanthe depth of the ablated spot area. Theoretically, the possibleparticles (if they would be generated) could now have a maximum size ofthe spot depth. The rotating scanner now accomplishes the production ofgood quality, particle free plasma with great production rate, withsimultaneously large scanning width, especially beneficial forsubstrates comprising large surface areas to be coated. Furthermore, theFIGS. 10 a, 10 b and 10 c clearly demonstrate that opposite to presenttechniques, the already ablated target material area can be ablated fornew generation of high class plasma—reducing thus radically the overallcoating/thin-film producing cost.

FIG. 11 demonstrates an example wherein coating is carried out byemploying a pico-second USPLD-laser and scanning the laser pulses withturbine scanner. Here, the scanning speed is 30 m/s, the laserspot-width being 30 μm. In this example, there is ⅓ overlapping betweenthe adjacent pulses.

Next some materials are described which are suitable as target materialsfor providing layers of the solar cell. The layer of conductivetransparent material can be made of e.g. indium tin oxide, aluminumdoped zinc oxide, tin oxide or fluorine-doped tin oxide. The layer ofconductive non-transparent material can be made of e.g. aluminum, copperor silver. The layer of semiconducting material can be made of e.g.silicon, germanium indium tin oxide, aluminum doped zinc oxide, tinoxide or fluorine-doped tin oxide. The layer of antireflective coatingcan be made of e.g. of silicon nitride or titanium oxide. However, thisare just some examples of commonly used materials. Next, some furtheralternatives are discussed in more detail.

Advantageous metal oxides include for example aluminum oxide and itsdifferent composites such as aluminum titan oxide (ATO). Due to itsresistivity, high-optical transparencies possessing high-quality indiumtin oxide (ITO) is especially preferred in applications wherein thecoating can be employed to warm-up the coated surface. It can also beemployed in solar-control Yttrium stabilized zirconium oxide is anotherexample of different oxides possessing both excellent optical,wear-resistant properties.

Some further metals can also be applied in solar cell applications.Here, the optical properties of metal-derived thin-films are somewhatdifferent from those of bulk metals. In ultrathin films (<100 Å thick)variations make the concept of optical constants problematic, thequality and surface roughness of the coating (thin film) being thuscritical technical features. Such coatings can easily be produced withthe method of present invention.

Dielectric materials employed in present applications include fluorides(e.g. MgF₂, CeF₃), oxides (e.g. Al₂O₃, TiO₂, SiO₂), sulfides (e.g. ZnS,CdS) and assorted compounds such as ZnSe and ZnTe. An essential commonfeature of dielectric optical materials in their very low absorption(α<10³/cm) in some relevant portion of the spectrum; in this region theyare essentially transparent (e.g. fluorides and oxides in the visibleand infrared, chalcogenides in the infrared). Dielectric coatings can beadvantageously produced with the method of present invention.

Transparent conducting films may consist either of very thin metals orsemiconducting oxides and/ and most presently even nitrides such asindium gallium nitride in front electrodes for solar cells.

Metals that have conventionally been employed be as transparentconductors include Au, Pt, Rh, Ag, Cu, Fe and Ni. Simultaneousoptimization of conductivity and transparency presents a considerablechallenge in film deposition. At one extreme are discontinuous islandsof considerable transparency but high resistivity; at the other arefilms that coalesce early and are continuous, possessing highconductivity but low transparency. For these reasons, thesemi-conducting oxides such as SnO₂, In₂O₃, CdO, and, more commonly,their alloys (e.g. ITO), doped In₂O₃ (with Sn, Sb) and doped SnO₂ (withF, Cl, etc.) are used.

The metal oxide coatings can be produced by either ablating metal ormetals in active oxygen atmosphere or by ablating oxide-materials. Evenin latter possibility, it is possible to enhance the coating qualityand/or production rate by conducting the ablation in reactive oxygen.When producing nitrides it is according to the invention possible to usenitrogen atmosphere or liquid ammonia in order to enhance the coatingquality. A representative example of invention is production of carbonnitride (C₃N₄ films).

According another embodiment of the invention, said uniform surface areaof solar cell layer is produced with carbon material comprising over 90atomic-% of carbon, with more than 70% of sp³-bonding. Such materialsinclude for example amorphous diamond, nano-crystalline diamond or evenpseudo-monocrystalline diamond. Various diamond coatings give the glassproduct excellent tribological, wear- and scratch-free properties butincrease also the heat-conductivity and -resistance. Diamond-coatings onglass can be used with special preference in solar cells, if of highquality, i.e. crystalline form.

In a still another embodiment of the invention, said uniform surfacearea of may be produced of material comprising carbon, nitrogen and/orboron in different ratios. Such materials include boron carbon nitride,carbon nitride (both C₂N₂ and C₃N₄), boron nitride, boron carbide orphases of different hybridizations of B—N, B—C and C—N phases. Saidmaterials are diamond-like materials having low densities, are extremelywear-resistant, and are generally chemically inert. For example carbonnitrides can be employed to protect glass products against corrosiveconditions, as coatings in solar cells.

According to one embodiment of invention, the outer surface of the solarcell product is coated with only one single coating. According toanother embodiment of the invention, said uniform surface of the solarcell is coated with multilayered coating. Several coatings can beproduced in for different reasons. One reason might be to enhance theadhesion of certain coatings to glass product surfaced by manufacturinga first set of coating having better adhesion to glass surface andpossessing such properties that the following coating layer has betteradhesion to said layer than to glass surface itself. Additionally, themultilayered coating can possess several functions not achievablewithout said structure. The present invention accomplishes theproduction of several coatings in one single coating chamber or in theadjacent chambers.

The present invention further accomplishes the production of compositelayers/coatings to solar cells ablating simultaneously one compositematerial target or two or more target materials comprising one or moresubstances.

A suitable thickness of an ablated layer is e.g. between 20 nm and 20μm, preferably between 100 nm and 5 μm. The coating thicknesses must notbe limited to those, because the present invention accomplishes thepreparation of molecular scale coatings on the other hand, very thickcoatings such as 100 μm and over, on the other hand.

According to the invention there may also provided a solar cell productcomprising a certain surface being coated by laser ablation wherein thecoated uniform surface area comprises at least 0.2 dm² and that thecoating has been carried by employing ultra short pulsed laserdeposition wherein pulsed laser beam is scanned with a rotating opticalscanner comprising at least one mirror for reflecting said laser beam.The benefits received with these products are described in more detailin the previous description of the method.

In a more preferred embodiment of the invention said uniform surfacearea comprises at least 0.5 dm². In a still more preferable embodimentof the invention said uniform surface area comprises at least 1.0 dm².The invention accomplishes easily also the products comprising uniformcoated surface areas larger than 0.5 m², such as 1 m² and over.

According to one embodiment of the invention the average surfaceroughness of produced coating on said uniform surface area is less than100 nm as scanned from an area of 100 μm² with Atomic Force Microscope(AFM).

According to another embodiment of the invention the opticaltransmission of produced coating on said uniform surface area is no lessthan 88%, preferably no less than 90% and most preferably no less than92%. In some cases the optical transparency can exceed 98%.

According to still another embodiment of the invention said uniformsurface area is coated in a manner wherein the first 50% of said coatingon said uniform surface area does not contain any particles having adiameter exceeding 1000 nm, preferably 100 nm and most preferably 30 nm.

According to one embodiment of the invention said layer comprises metal,metal oxide, metal nitride, metal carbide or mixtures of these. Thepossible metals were described earlier in description of now inventedcoating method.

According to another embodiment of the invention said uniform surfacearea of glass product is coated with carbon material comprising over 90atomic-% of carbon, with more than 70% of sp³-bonding. The possiblecarbon materials were described earlier in description of now inventedcoating method.

According to still another embodiment of the invention said uniformsurface area comprises carbon, nitrogen and/or boron in differentratios. Such materials were described earlier in description of nowinvented coating method.

According to still another embodiment of the invention said uniformsurface area the product is coated with organic polymer material. Suchmaterials were described earlier in more detail in description of nowinvented coating method.

According to one preferred embodiment of the invention the thickness ofsaid coating on uniform surface of glass product is between 20 nm and 20μm, preferably between 100 nm and 5 μm. The invention accomplishes alsocoated glass products comprising one or several atomic layer coatingsand thick coatings such as exceeding 100 μm, for example 1 mm.

In this patent specification the structure of the other variouscomponents of a laser ablation apparatus is not described in more detailas they can be implemented using the description above and the generalknowledge of a person skilled in the art.

Above, only some embodiments of the solution according to the inventionhave been described. The principle according to the invention cannaturally be modified within the frame of the scope defined by theclaims, for example, by modification of the details of theimplementation and ranges of use.

For example, only a few structures of solar cells have been discussed asexamples. There are many other types of other alternative structures,wherein the structure comprises one or several layers of differentmaterials, commonly semiconducting, conductive, insulating andtransparent materials. In is naturally possible to use to apply thepresent invention also in such other types of structures of solar cells.

1-41. (canceled)
 42. A method for producing by laser ablation at leastone layer having a surface and to be used as part of a solar cell,characterized in that the surface area to be produced comprises at leastan area 0.2 dm² and the coating is performed by employing ultra shortpulsed laser deposition wherein pulsed laser beam is scanned with arotating optical scanner comprising at least one mirror for reflectingsaid laser beam.
 43. A method according to claim 42, characterized inthat said surface area is a uniform surface area.
 44. A method accordingto claim 42, characterized in that said surface area comprises at leastan area 0.5 dm², preferably at least an area 1.0 dm².
 45. A methodaccording to claim 42, characterized in that the employed pulsefrequency of said laser deposition is at least 1 MHz.
 46. A methodaccording to claim 42, characterized in that the surface of thesemiconductor or conductive layer does not comprise short circuitingdefect factors.
 47. A method according to claim 42, characterized inthat the average surface roughness of produced layer on said uniformsurface area is less than 100 nm as scanned from an area of 100 μm² withAtomic Force Microscope (AFM).
 48. A method according to claim 42,characterized in that the optical transmission of a produced layer onsaid uniform surface area is no less than 88%, preferably no less than90% and most preferably no less than 92%.
 49. A method according toclaim 42, characterized in that a layer of conductive transparentmaterial is made of indium tin oxide, aluminum doped zinc oxide, tinoxide or fluorine-doped tin oxide.
 50. A method according to claim 42,characterized in that a layer of conductive non-transparent material ismade of aluminum or copper.
 51. A method according to claim 42,characterized in that a layer of semiconducting material is made ofsilicon, germanium indium tin oxide, aluminum doped zinc oxide, tinoxide or fluorine-doped tin oxide.
 52. A method according to claim 42,characterized in that a layer of antireflective coating is made ofsilicon nitride or titanium oxide.
 53. A method according to claim 42,characterized in that the layer comprises at least one of followingmaterials or a mixture of these: at least 80% metal oxide or compositionthereof; or carbon, nitrogen and/or boron; or carbon material comprisingover 90 atomic-% of carbon, with more than 70% of sp³-bonding.
 54. Amethod according to claim 42, characterized in that the outer surface ofsaid solar cell is coated with multilayered coating.
 55. A methodaccording to claim 42, characterized in that the thickness of the layeris between 20 nm and 20 μm, preferably between 100 nm and 5 μm.
 56. Asolar cell comprising at least one layer with a surface, produced bylaser ablation, characterized in that the surface area to be producedcomprises at least an area 0.2 dm² and the layer has been produced byemploying ultra short pulsed laser deposition wherein pulsed laser beamis scanned with a rotating optical scanner comprising at least onemirror for reflecting said laser beam.
 57. A solar cell according toclaim 56, characterized in that said surface area is a uniform surfacearea.
 58. A solar cell according to claim 56, characterized in that saidsurface area comprises at least an area 0.5 dm², preferably at least anarea 1.0 dm².
 59. A solar cell according to claim 56, characterized inthat the average surface roughness of produced coating on said uniformsurface area is less than 100 nm as scanned from an area of 100 μm² withAtomic Force Microscope (AFM).
 60. A solar cell according to claim 56,characterized in that the surface does not comprise short circuitingdefect factors.
 61. A solar cell according to claim 56, characterized inthat the optical transmission of produced coating on said uniformsurface area is no less than 88%, preferably no less than 90% and mostpreferably no less than 92%.
 62. A solar cell according to claim 56,characterized in that said surface area is coated in a manner whereinthe first 50% of said coating on said uniform surface area does notcontain any particles having a diameter exceeding 1000 nm, preferably100 nm and most preferably 30 nm.
 63. A solar cell according to claim56, characterized in that a layer of conductive transparent material ismade of indium tin oxide, aluminum doped zinc oxide, tin oxide orfluorine-doped tin oxide.
 64. A solar cell according to claim 56,characterized in that a layer of conductive non-transparent materialcomprises aluminum, copper or silver.
 65. A solar cell according toclaim 56, characterized in that a layer of semiconducting materialcomprises silicon, germanium indium tin oxide, aluminum doped zincoxide, tin oxide or fluorine-doped tin oxide.
 66. A solar cell accordingto claim 56, characterized in that a layer of antireflective coatingcomprises silicon nitride or titanium oxide.
 67. A solar cell accordingto claim 56, characterized in that said layer comprises at least one offollowing materials or a mixture of these: metal, metal oxide, metalnitride, and/or metal carbide; or carbon, nitrogen and/or boron; orcarbon material comprising over 90 atomic-% of carbon, with more than70% of sp³-bonding.
 68. A solar cell according to claim 56,characterized in that the outer surface of the solar cell has amultilayered coating.
 69. A solar cell according to claim 56,characterized in that the thickness of said layer for a solar cell isbetween 20 nm and 20 μm, preferably between 100 nm and 5 μm.
 70. Anarrangement for producing at least one part of a solar cell, theapparatus comprising means for producing at least one layer with asurface by laser ablation, characterized in that the surface area to beproduced comprises at least an area 0.2 dm² and the arrangementcomprises means for providing the layer by employing ultra short pulsedlaser deposition wherein the arrangement comprises a rotating opticalscanner for scanning pulsed laser beam, the rotating scanner comprisingat least one mirror for reflecting said laser beam.
 71. An arrangementaccording to claim 70, characterized in that said surface area is auniform surface area.
 72. An arrangement according to claim 70,characterized in that the arrangement comprises means for producing atleast two layers for a same solar cell within a same chamber.
 73. Anarrangement according to claim 70, characterized in that the arrangementcomprises means for machining layers or substrate of a same solar cellwithin a same chamber.